With the remarkable progress of satellite communication technique in recent years, the satellite communication has become vigorously utilized in various applications such as satellite broadcasting, public communication, and the like. With such a trend, there have been earth stations having various sizes and structures ranging from a large scale station for public communication which is provided with an antenna of 10 meters in diameter to a domestic satellite broadcast receiver.
A large scale station, in general, employs an arrangement in which the communication apparatus placed within the station house is connected with the antenna site by a transmission line. A schematic arrangement of a transmission system in such an earth station is shown in FIG. 1.
Referring to FIG. 1, the antenna site 1 is connected with the in-station apparatus 2 through the transmission line 3. Since the carriers received from the in-station apparatus 2 is subject to level reduction stemming from the transmission line 3, the antenna site 1 is provided with an automatic level control (ALC) circuit for controlling the carrier level so as to compensate the received carriers for the level reduction. Further transmission processing, such as conversion of the transmission frequency, is carried out, and the resulting carriers are amplified in power by means of the HPA (high power amplifier) to emit from the antenna toward the satellite.
Typically, such an earth station is necessary to combine a plurality of carriers for emitting them from a single antenna simultaneously. In the in-station apparatus 2, a multi-carrier controller 4 is comprised of a carrier combiner 5 for combining the carriers received from the burst carrier transmitters (BWT) with the continuous carrier transmitters (CWT). The combined carriers are transmitted to the ALC of the antenna site 1 through the transmission line 3 together with the calculated combined level of the carriers.
The burst carrier and the continuous carrier mentioned above mean a short isolated sequence of modulated carriers and a continuous modulated carrier, respectively.
The multi-carrier controller which combines various transmission carriers and calculates the combined level is disclosed in Japanese Patent Laid-Open No. 3-71731. Using the transmission time of each carrier which is measured for each period of time, the controller performs automatic power control such that the transmission output power is kept stable with the number of the transmission carriers varying.
According to the foregoing conventional transmission level control system, all the received carriers combine in the carrier combiner 5 to be transmitted to the ALC apparatus through the transmission line 3 regardless of whether a received carrier is a burst carrier or a continuous carrier, and based on that combined level, the automatic level control is performed by the ALC apparatus in the antenna site 1. However, since the burst carrier instantaneously varies in level, the level detection and the computation for level control become extremely complicated, resulting in complication of the level control operation and then a costly control system.